mount: restore a shared group for propagated mounts
mount: restore a shared group for propagated mounts

When a mount is propagated, the result mount is always shared.
If we want to get a private mount, we need to convert it.

Cc: Kirill Tkhai <e5b39230c99b4d3ebd6aa54f6d823f95134bc39d@virtuozzo.com>
Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,AuthenticEshkinKot/criu,eabatalov/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,efiop/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,eabatalov/criu,efiop/criu,eabatalov/criu,eabatalov/criu,efiop/criu,efiop/criu,eabatalov/criu,KKoukiou/criu-remote,efiop/criu